When performing an Optical Time-Domain Reflectometer (OTDR) measurement on a Device Under Test (DUT), the modal distribution of the test light must be carefully controlled in order to measure reproducible values of insertion loss or attenuation. If the modal distribution of the test light is not well controlled, “differential mode attenuation” may lead to unrepeatable and irreproducible measurement results.
In order to address this issue, test and measurement international Standards such as the Telecommunication Industry Association (TIA-526-14-B) and the International Electrotechnical Commission (IEC 61280-4-1) define requirements on the modal distribution of test light for performing measurements on multimode DUTs. For example, the IEC 61280-4-1 Standard provides for a target for an Encircled Flux function, EF(r) characterizing the modal distribution of test light exiting the launch cord (into air) which is to be launched into the multimode DUT. The encircled flux function EF(r) defines very tight tolerances on deviations from that target. It is defined as that proportion of the total exiting optical power which falls within a circle (i.e. “encircled”) of radius r at the end face of the fiber, where r is the radial distance from the optical center of the fiber core, at four or five predefined radial values in the fiber core and for each of two wavelengths, i.e. 850 and 1300 nm. Other methods of characterizing the modal distribution of light also exist.
When light is coupled into a multimode launch cord, depending on the coupling conditions and on the optical power density of the optical source, the coupling may result in light exiting the multimode launch cord being “underfilled” (i.e. too few modes are excited) or “overfilled” (i.e. too many modes are excited). The modal distribution of the test light launched into the multimode DUT need to be adjusted to comply with the EF requirements or any other suitable modal distribution requirement that may be defined by Standards. Various means for controlling the modal distribution of the test light exist in the art including that based on mandrel wrapping, i.e. the tight winding of a multimode optical fiber about a circular mandrel of a given diameter. Mandrel wrapping results in a preferential attenuation of the high-order modes corresponding to an initially overfilled condition. Launch conditions in compliance with a given set of modal distribution requirements (e.g. as defined in the IEC 61280-4-1 Standard) are said to be “mode conditioned”.
In some instances, the multimode DUT may be provided in the form of a multimode multi-fiber array DUT, e.g., a multi-fiber ribbon link or a multi-fiber device terminated by multi-fiber array connectors. When performing the OTDR measurement on the multi-fiber array DUT, the OTDR measurement is manually and sequentially repeated for each fiber of the multi-fiber array DUT. In order to access each fiber path of the multi-fiber array DUT, a fan out adapter may be connected between the launch cord and the multi-fiber array connector.
Although existing measurement systems were satisfactory to a certain extent, there remains room for improvement, particularly in providing an automated method of performing an OTDR measurement on the multi-fiber array DUT while maintaining launch conditions that are in compliance with relevant modal distribution requirements. Moreover, there also remains room for improvement in increasing the life span of test components used in performing an OTDR measurement on a multimode multi-fiber array DUT.